Sustainable Synthesis of PPHC@TiO₂/PPy Composite for Enhanced Congo Red Dye Removal: Kinetics, Isotherms, and Mechanistic Insights

Abstract

The growing environmental impact of synthetic dyes like Congo Red (CR) necessitates the development of efficient and sustainable adsorbents for water treatment. This study presents the green synthesis of a novel ternary composite, PPHC@TiO₂/PPy, by integrating pomegranate peel-derived hydrochar (PPHC), titanium dioxide nanoparticles (TiO₂ NPs), and polypyrrole (PPy) through hydrothermal carbonization, TiO₂ precipitation, and in situ oxidative polymerization. The composite was characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) surface area analysis, and zeta potential measurements. These analyses confirmed the formation of a mesoporous hybrid material with favorable surface chemistry, thermal stability, and a point of zero charge (pHₚzc) of 9.93. Under optimized adsorption conditions (50 ppm CR, 25 °C, 50 mg dosage, 360 min), the composite exhibited a high adsorption capacity of 356.61 mg/g. Kinetic data conformed to a pseudo-second-order model, and equilibrium results followed the Langmuir isotherm, indicating monolayer chemisorption. Thermodynamic parameters (ΔH° = −34.26 kJ/mol; ΔG° = −27.62 to − 27.17 kJ/mol) confirmed the spontaneous and exothermic nature of the process. These results demonstrate the strong potential of PPHC@TiO₂/PPy as a high-performance, eco-friendly adsorbent for the effective removal of anionic dyes in wastewater treatment applications.

Graphical Abstract